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STORY: Biden and Yoon said their countries' decades-old alliance needed to develop not only to face North Korean threats but also to keep the Indo-Pacific region "free and open" and protect global supply chains.

The friendly encounter between allies is clouded by intelligence showing North Korean leader Kim Jong Un is prepared to conduct nuclear or missile tests.

Yoon, keen for South Korea to play a bigger role in regional issues, is expected to make his country one of the inaugural members of Biden's Indo-Pacific Economic Framework (IPEF), which will be announced during the trip to set standards on labor, the environment and supply chains.

The South Korean president has also signaled a tougher line on North Korea than his predecessor and is expected to ask for Biden's help. Yoon has warned of a preemptive strike by South Korea if there is a sign of an imminent attack and vowed to strengthen the South's deterrent capability.